Make ozone/drm/mojo more immune to startup races

ui/ozone/platform/drm/gpu/drm_thread.cc

Index: ui/ozone/platform/drm/gpu/drm_thread.cc
diff --git a/ui/ozone/platform/drm/gpu/drm_thread.cc b/ui/ozone/platform/drm/gpu/drm_thread.cc
index 8eaaeee1deb0a83efeccac5a07ad8781784f111c..1dd66d870da6b3490da874fcc504591cd9a1492f 100644
--- a/ui/ozone/platform/drm/gpu/drm_thread.cc
+++ b/ui/ozone/platform/drm/gpu/drm_thread.cc
@@ -207,8 +207,10 @@ void DrmThread::SetCursor(const gfx::AcceleratedWidget& widget,
                           const std::vector<SkBitmap>& bitmaps,
                           const gfx::Point& location,
                           int32_t frame_delay_ms) {
-  screen_manager_->GetWindow(widget)
-      ->SetCursor(bitmaps, location, frame_delay_ms);
+  auto window = screen_manager_->GetWindow(widget);

[dnicoara, 2017/05/26 15:51:04]

Why is this needed? The window lifetime is "managed" by the browser process via
CreateWindow()/DestroyWindow(). It should never be the case that a window event
arrives before the window is created or after the window is destroyed.

+  if (!window)
+    return;
+  window->SetCursor(bitmaps, location, frame_delay_ms);
 }
 
 void DrmThread::MoveCursor(const gfx::AcceleratedWidget& widget,